1. Field of the Invention
The present invention relates to a charged particle beam writing method and a charged particle beam writing apparatus.
2. Background Art
Electron beam writing apparatuses have been used to write a micropattern on a sample such as a mask substrate or a semiconductor wafer. When writing on a sample using such an apparatus, it is necessary to accurately measure the height of the surface of the sample before the write operation in order to avoid displacement of the electron beam from the target writing position, as well as to avoid defocus of the electron beam. Specifically, before the write operation, the height measuring unit in the electron beam writing apparatus emits light over the region to be irradiated, detects the reflected light, and thereby determines the height of the sample surface. The electron beam writing apparatus then adjusts, in accordance with the determined height, lenses for focusing the electron beam onto the sample surface, for example, thereby accurately focusing the beam onto the sample surface.
If the surface of the sample is perfectly flat, the desired pattern can be written thereon by measuring the height of any arbitrary point on the surface and directing and focusing the electron beam only at this height when writing anywhere on the surface. However, the surface of actual samples is not perfectly flat and is slightly deformed or warped. Therefore, if the electron beam is continuously focused only at a single height (which was determined at a selected point on the sample surface) when writing on the entire writing region on the sample surface, there will result displacement of the electron beam from desired writing positions as well as defocus of the electron beam.
To solve this problem, Japanese Laid-Open Patent Publication No. 61-34936 (1986) discloses a method of correcting displacement of the electron beam from desired writing positions and defocus of the electron beam due to deformation of the sample. According to this method, the height of several points on the sample surface is measured, and then a plurality of quadratic equations are formed using the measurement results. The coefficients of these quadratic equations are then determined by the least squares method. The resulting equation with the determined coefficients (a correction equation) is used to correct the focus of the electron beam and the deflection gain.
Thus, the method described in the above patent publication determines the height of every point on the sample surface by interpolation using height measurements of several points on the sample surface. In order for this interpolation to be effective, the measurement at these selected points must be accurate. It should be noted that a Z sensor is generally used for this measuring purpose. For example, an L-shaped moving mirror is mounted at one end of the XY stage on which the sample is placed, and a laser interferometer is disposed to face the mirror surface of the moving mirror. The laser interferometer is adapted to measure the XY coordinates of the XY stage and the angle of rotation of the XY stage in the X-Y plane by using the laser light reflected from the moving mirror. The measurement results are output to the control system which then outputs a control signal to the driving unit based on the measurement results to adjust the position of the XY stage in the X-Y plane. The control system also outputs a control signal to the Z sensor to measure the coordinate position of the sample in the Z-axis direction. It will be noted that the Z sensor is an optical position detector including a projector and a photodetector and uses an obliquely incident beam. The detection beam emitted downward at an angle from the projector impinges on the surface of the sample and is reflected upward at an angle to enter the photodetector. The parameters of this reflected beam incident on the photodetector are measured to detect the height of the sample surface.
It has been found, however, that the above measuring method is liable to measurement errors due to lack of intensity of the laser beam incident on the laser interferometer and due to poor coupling between the laser interferometer and the Z sensor. These measurement errors prevent the creation of a proper height distribution map based on the measurements by the Z sensor; that is, these errors may cause a distortion in the shape of the map or even prevent the preparation of any map.
The present invention has been made in view of the above problems. It is, therefore, an object of the present invention to provide a charged particle beam writing method and a charged particle beam writing apparatus capable of accurately focusing a charged particle beam onto the surface of a sample even when measurement errors occur by creating a map and performing the focusing operation in accordance with this map.
Other challenges and advantages of the present invention are apparent from the following description.